Plastic pipes are widely used in various engineering fields. For its advantages of no contamination of carried media, resistance to corrosion, light in weight, and convenience in transportation and installation, the prospect of the application thereof is becoming even more brilliant.
However, the conventional plastic pipe is restricted in some industrial fields due to its poor performance in mechanic strength, rigidity and heat resistance. So far, many forms of composite steel-plastic pipes, as well as the manufacturing methods and devices, have been proposed, in order to improve the mechanic strength, rigidity and heat resistance of plastic pipes to be applied to the fields with higher pressure. For example, U.S. Pat. No. 3, 526, 692 granted to Osaka, U.S. Pat. No. 4, 657, 049 to Forty et al. and U.S. Pat. No. 4, 167, 953 to Carlstrom et al. have disclosed methods for fabricating reinforced plastic pipes with spiral steel wires to improve the strength thereof. The apparatus for manufacturing reinforced plastic composite pipes has been provided by U.S. Pat. No. 4, 017, 232(Holden et al.) and U.S. Pat. No. 4, 135, 869 (Loyer et al.).
The method for fabricating the conventional composite plastic pipes is, as disclosed by U.S. Pat. No. 3, 526, 692, comprises the following steps: forming a spiral steel wire; inserting the spiral steel wire into a plastic extrusion mold in the direction of the axis; forming a plastic pipe body in mold so that the spiral steel wire is embedded in the plastic pipe body and protruded outwardly from the outer surface of the plastic pipe body; and putting the resulted structure through a second extrusion mold so that a plastic layer is adhered to the periphery of the plastic body to cover the protruding spiral steel wire. There are two drawbacks in this method. Firstly, the process is complicated, since twice extrusions of plastic have to be performed to get a desired product. Secondly, What is embedded in the plastic pipe is a spiral steel wire. Although the measure has reinforced the plastic pipe to a certain extent, the effect is not satisfactory.
The Chinese patent ZL94104509.9 held by the inventor of the present invention provides another method and device for fabricating a netted steel wire skeleton--composite plastic pipe, which has overcome the drawbacks of insufficient strength and complicated process in the prior art. The commercialized product has been obtained by the patented method. The compressive strength of the product is from 1 to 6.5 Mpa. This patent is incorporated herein as reference.
The inventor has discovered on the basis of study that the composite netted steel wire skeleton--plastic pipe made according to the Chinese patent ZL94104509. 9 have also some shortcomings, though it is obviously superior to other similar products. The netted steel wire skeleton, according to the patent, is configured by intersecting longitude lines and latitude lines. Both take 50% respectively. As matter of fact, the longitudes have little effect in bearing interior pressure or reinforcing rigidity. The effect for increasing strength by steel wire skeleton has been reduced by about one third. In the other hand, since longitudes have almost nothing to do with bearing interior pressure, the diameter of the latitudes has to be increased to enhance the strength of the pipes. Large diameter latitudes result in inevitably a thicker pipe wall, thus increasing the production cost. Moreover, the purpose of welding is to secure the two groups of steel wires to form a netted skeleton. This results in he manufacturing device with a complicated structure, great dimension, and large energy consumption.